This invention relates to gasification systems and processes. In particular the invention relates to a low-temperature system and process for removing solids byproducts of a gasification process.
Gasification is a process for the production of power, chemicals, and industrial gases from carbonaceous or hydrocarbon feedstocks such as coal, heavy oil, and petroleum coke. Gasification converts carbonaceous or hydrocarbon feedstocks into synthesis gas, also known as syngas, comprising primarily hydrogen and carbon monoxide. The resultant syngas is a feedstock for making useful organic compounds or can be used as a clean fuel to produce power.
In a typical gasification plant, a carbonaceous or hydrocarbon feedstock and molecular oxygen are contacted at high pressures within a partial oxidation reactor (gasifier). The feedstock and molecular oxygen react and form syngas. Non-gasifiable ash material and unconverted and/or incompletely converted feedstock are by products of the process and take essentially two forms: molten slag and smaller particles referred to as “fines”. A quench process cools and saturates the syngas and also causes the molten slag to be rapidly cooled and solidified into irregularly shaped particles of varying size in a quench chamber. The slag particles are separated from the syngas, and are captured in the quench chamber.
Coarse slag flows by gravity from the quench section of the gasifier into a pressurized lockhopper at regular intervals. Afterwards, the lockhopper is automatically isolated from the gasifier, depressured, and emptied onto a drag conveyor. Water in the lockhopper is replaced with grey water from a lockhopper flush drum and the system is repressurized. The slag is screened to reduce moisture, and sent to disposal offsite. The reclaimed water is sent to the vacuum flash drum in the black water flash section of the plant.
A separate system is used to remove fines from the system after quenching. The quench water containing fines is referred to as black water. A black water flash section concentrates solids and removes dissolved syngas from the black water. Flashing is a process by which the black water passes through a flash valve into a flash drum which is of lower pressure. The pressure reduction causes the black water to be partially evaporated and cooled. To achieve the desired amount of cooling, black water is flashed in two stages, a low-pressure flash and a vacuum flash. The cooled black water is sent a filtration step. Cooling the black water to under 200 F allows the recycling of the fines in a slurry grinding mill for potential reuse as gasifier feedstock.
The use of a separate system for the fines removal increases maintenance costs and decreases gasifier availability. The flash valve conditions are erosive requiring expensive valves that must be frequently replaced. In addition, scale formed in a syngas scrubber, which flows through the black water flash section plugs the small openings in the flash valves requiring gasifier shutdowns or potentially unsafe purges to atmosphere.
Additionally, extra equipment is required to implement this system. Separate heat exchangers in which the resultant vapor from each of the flash stages must be condensed, and condensate collection and recycle systems. Also, a vacuum system is required for the vacuum flash stage. Furthermore, the black water lines are in solids service and must be sloped downward causing increased gasifier and scrubber height requirements.
What is needed is a combined low-temperature system for removing slag and fines resulting from gasification that reduces maintenance costs and plant capital costs, increases plant availability, and reduces plant scrubber structure height.